The grading of dielectric insulations for electrical cables for relatively high voltage service comprising the introduction of predetermined gradations of dielectric characteristics in a body or unit of dielectric insulation enclosing an electrical conductor is an old concept and subject in the electrical art. For instance, various aspects and means of grading electrical insulations for cable are proposed and/or disclosed in a paper entitled "Silicone Rubber Graded Construction For High Voltage Insulation," by S. J. Nizinski, published in Wire and Wire Products, Volume 3, No. 5, May, 1962, page 628 et seq., and in British Patent 1568 of 1901 and the following U.S. Pat. Nos. 1,802,030; 2,123,746; 2,198,977; 3,160,703; 3,287,489; 3,433,891; 3,711,631 and 3,869,621.
The disclosures and contents of the foregoing publication and patents of the prior art are incorporated herein by reference.
The grading of electrical insulations, as is evident from the foregoing prior art, generally comprises providing an insulation including a series of at least two contiguous sections or areas of different specific inductive capacitance values. An insulation embodying a sequence of different specific inductive capacitance values with the highest specific inductive capacitance closest to the electrical conductor and successively reduced values therefrom, incurs more uniform or evenly distributed electrical stresses or voltage gradients therein when subjected to high voltage alternating electrical current.
However, unlike alternating current electrical systems for cable insulation wherein the maximum degree of electrical stress occurs at the surface of the dielectric insulation adjoining or closest to the conductor carrying the alternating current and progressively diminishes outwardly therefrom, in direct current electrical systems the stress or voltage gradient is distributed resistivity across the thickness of the insulation. Also, distinct from alternating current systems wherein the electrical stresses are nearly independent of temperature conditions, the resistivity of polymeric materials or insulations thereof in direct current transmitting cable is dependent upon temperature, and other conditions including electrical stress or voltage gradient and time. For example, as an electrical cable heats up to operating temperature, or increases in temperature due to external or ambient conditions, the stress conditions across the insulation progressively increase within the outermost regions of the insulation and correspondingly progressively decrease within the innermost regions of insulation adjoining the conductor, whereby the maximum stress exists within the insulation farthest from the electrical conductor and the minimum stress exists within the insulation closest to the conductor. See an article entitled "Electrical Stress Distribution In High Voltage DC Solid Dielectric Cables" by C. R. Mc Cullough, published in IEEE 6866-EI-67.